Methods of preparing fibrous metal materials and to materials prepared thereby

ABSTRACT

THE INVENTION PROVIDES A METHOD OF PREPARING A FIBROUS METAL MATERIAL, WHICH COMPRISES, FIRSTLY, COVERING AN ELECTRICALLY CONDUCTIVE CARBON SKELETON, HAVING A GENERAL SHAPE CORRESPONDING TO THAT OF THE FINISHED MATERIAL WHICH IT IS DESIRED TO OBTAIN, WITH A DEPOSIT OF A SLIGHTLY ELECTROPOSITIVE METAL OR ALLOY, THE ELECTROPOSITIVITY NOT BEING GREATER THAN 0.7 AND, SECONDLY, ELIMINATING THE AFORE-MENTIONED SKELETON BY OXIDATION AT A HIGH TEMPERATURE LEAVING THE DEPOSITED METAL OR ALLOY ONLY, HAVING A FIBROUS TEXTURE.

United States Patent Office 3,695,869 Patented Oct. 3, 1972 3,695,869 METHOD F PREPARING FIBROUS METAL MATELS AND TO MATERIALS PRE- PARED THEREBY Andre Hivert, Pontoise, Pierre Lepetit, Saint-Vrain, and Andre Walder, Bourg-la-Reine, France, assignors to Ofiice National dEtudes et de Recherches, Chatillonsous-Eagnenx, France N0 Drawing. Filed Sept. 18, 1970, Ser. No. 73,613 Claims priority, application France, Sept. 23, 1969, 6932391 Int. Cl. 1322f 1/00 US. Cl. 75211 15 Claims ABSTRACT OF THE DISCLOSURE The invention provides a method of preparing a fibrous metal material, which comprises, firstly, covering an electrically conductive carbon skeleton, having a general shape corresponding to that of the finished material which it is desired to obtain, with a deposit of a slightly electropositive metal or alloy, the electropositivity not being greater than 0.7 and, secondly, eliminating the afore-mentioned skeleton by oxidation at a high temperature leaving the deposited metal or alloy only, having a fibrous texture.

This invention relates to a method of preparing fibrous metal materials and to materials prepared thereby, that is simple fibres or more highly-worked products. The expression fibrous metal material is used herein in a very general sense and applies both to simple metal fibres of pure metal or alloys and to more highly-worked products obtained by interlocking the metal fibres (for example metal felts) or by weaving them (for example metal fabrics). Such more highly-worked products have special properties (for example a flexible shape, mechanical strength, high porosity, a large surface per unit volume, high electric conductivity, low specific heat, and low heat conductivity) which generally make them particularly suitable for use in the manufacture of highquality industrial apparatus such as electrolyte supports for fuel cells or accumulators, heat insulation devices, abradable seals, heating resistances, or filters for gases at high temperatures or corrosive liquids.

Fibrous metal materials are known, based inter alia on nickel, but the known materials have a non-uniform structure which reduces their mechanical strength and are also expensive to produce. Furthermore, the known materials have poor resistance to oxidation, more particularly at high temperatures. For example, a prior-art nickel felt becomes completely oxidised (that is, is completely converted into nickel oxide) in less than 100 hours at a temperature of 800 C.

For many industrial applications, therefore, it is desirable to improve the uniformity and reduce the cost of the afore-mentioned materials. Furthermore, the materials operate under conditions in which they are frequently liable to corrosion (more particularly at high temperatures) which is all the more dangerous in that the materials are porous and any oxidation which occurs will not be limited to their outer surface but will penetrate through their entire mass.

It can, therefore, be seen that it is desirable to improve the uniformity of the afore-mentioned materials (thus improving their mechanical strength), to reduce their cost and, in many cases, to increase their resistance to corrosion, more particularly by oxidation at high temperatures.

An object of the present invention is to provide a method of preparing improved fibrous metal materials and materials prepared thereby.

According to the present invention there is provided a method of preparing a fibrous metal material which comprises, firstly, covering an electrically conductive carbon skeleton, having a general shape corresponding to that of the finished material (for example, fibres, felt or ribbons or sheets of fabric) which it is desired to obtain, with a deposit of a slightly electropositive metal or alloy, the electropositivity, which is not greater than 0.7, enabling electrolytic or chemical deposition methods to be used and preventing the formation of stable carbon compounds of the deposited metals, and, secondly, eliminating the afore-mentioned skeleton by oxidation at a high temperature leaving the deposited metal or alloy only, having a fibrous texture. If required, the material obtained is subject to vapor treatment with at least one metal element which is introduced inside the material so as to increase its corrosion resistance or improve its mechanical properties.

The resulting materials generally have increased mechanical strength and flexibility, are cheaper and have improved resistance to corrosion, more particularly by oxidation at high temperatures.

The conductive carbon skeleton is prepared either from carbon elements joined by conductive pyrolytic bridges formed by heat treatment at approximately SOD-850 C. in a neutral atmosphere (inter alia nitrogen or argon) containing a small proportion (for example of the order of 1 to 5% by volume) of a hydrocarbon, advantageously xylene, having a vapour pressure under atmospheric conditions which automatically gives the required partial pressure when the neutral carrier gas is bubbled therethrough, or is prepared from elements containing a sufficiently high proportion of carbon to leave a residue of at least 20% by weight of carbon after pyrolysis treatment in a neutral atmosphere, the carbon-containing elements in practive being pyrolytically treated in a neutral atmosphere which, as before, contains a small proportion of a hydrocarbon such as xylene.

The slightly electropositive metal or alloy is advantageously deposited on the conductive carbon skeleton by a chemical or electrolytic method. The metal or alloy used for the deposit is preferably nickel, cobalt, possibly iron, copper, silver, or an alloy of a two or more of the afore-mentioned materials.

The oxidation operation at a high temperature for eliminating the skeleton is preferably performed either in a hydrogen atmosphere containing a suitable proportion of water vapour such that the carbon is burnt without the metal being oxidised, the treatment being performed at a temperature of from 800 to 1100 C. for from 20 to 5 hours, or in air at a temperature of from 500 to 700 C. for from 20 to 5 hours, so that the carbon is effectively eliminated Without excessive oxidation of the metal deposit.

In the case of a mechanically worked product (in contrast to simple isolated fibres), more particularly a mechanically worked product made of interlocking fibres, the product is preferably sintered so as to consolidate the structure by codiifusion bonds at the points of contact. When the oxidation operation at a high temperature is performed in a hydrogen atmosphere, the afore-mentioned sintering occurs automatically during the high-temperature operation. On the other hand, when the oxidation operation at a high temperature occurs in air, it should be followed by sintering or de-oxidation treatment in a hydrogen atmosphere.

Alternatively, when the carbon skeleton is made up of non-interlocking fibres, the metal fibres obtained may be 3 felted by the same treatment as used in paper-making, before the carbon skeleton is eliminated.

Finally, when necessary and for the purposes mentioned hereinbefore, the fibrous metal material thus obtained may be given further treatment in order to produce a further improvement in its properties, inter alia its resistance to oxidation at a high temperature and its mechanical strength. The further treatment may advantageously be a cold or hot mechanical operation, as rolling, or a treatment with chromium, chromium and aluminium, or possibly chromium-aluminium-titanium or another complex alloy of the same kind, the afore-mentioned further treatment preferably being performed by methods previously devised by the applicants.

The following examples illustrate the invention, being directed, by way of example to the manufacture of metal felts.

EXAMPLE 1 Crude (non-carded) cotton was placed in a box having a cover admitting a limited flow of air, and was first heated to a temperature of about 500 C. in order to eliminate most of the volatile substances. After cooling, the box was placed in a furnace in a flow of nitrogen previously saturated with xylene under atmospheric conditions, and was heated to a temperature of about 800 C. for about one hour. The resulting carbon fibres were cut into small pieces measuring a few milimetres and disposed in a solution containing nickel acetate, glycolic acid, hydrazine and disodium ethylene diaminetetraacetate. The mixture was heated for from 1 to 2 hours, depending on the desired thickness of the deposit. The fibres were then washed and dried, followed by heating in air to a temperature of about 600 C. for about 10 hours. They were then poured into an alumina boat and sintered in hydrogen at a temperature of about 1,000 C. for about 1 hour.

The resulting nickel felt had the same shape as the boat. It could be rolled to obtain the required thickness and density, and could be molded.

EXAMPLE 2 As in Example 1, crude cotton was pyrolysed and the resulting carbon wadding was made electrically conductive by heating to a temperature of from 800 to 1000 C. in a furnace under a protective atmosphere of nitrogen saturated with xylene at ambient temperature. The wadding was broken into fragments and treated in an electrolytic drum (a rotating drum used for nickel and chromiumplating in screw and nut-and-bolt manufacture). The resulting carbon fibres were coated with nickel and suspended in the electrolyte of Example 1.

The felt was then formed by a process similar to those used in paper manufacture (for example by sieve draining) so as to obtain a cake having the same shape as the bottom of the sieve.

The cake was treated with air at a temperature of from 500 to 700 C. in a furnace for between 20 and 5 hours, thus completely eliminating the carbon and partly oxidising the nickel. Finally, the nickel oxide formed was reduced by treating the felt in a hydrogen atmosphere at a temperature of about 1,000 C. for about half an hour. The resulting felt had a very low density (of the order of 0.20) but could be made more dense if required by mechanical treatment.

EXAMPLE 3 A carbon skeleton was prepared from previously-manufactured carbon felt by heating to a temperature of approximately 700 C. in an argon atmosphere which had previously been saturated with xylene under atmospheric conditions in order to produce pyrolytic bridges which made the carbon felt electrically conductive.

Nickel was then deposited on the skeleton by electrolysis in a conventional bath, the skeleton being vibrated at approximately 50 Hz. The skeleton was divided into two parts and one sample of skeleton was then eliminated by oxygen treatment similar to that of Example 1, the other sample by that of Example 2. A satisfactory product was obtained in each case.

EXAMPLE 4 Carbon felt was prepared from carded cotton spread out in the amount of approximately 10 g. per drn. in a refractory steel box. (A number of layers can be superposed, depending on the height of the box, provided that each is separated by a sheet of paper.) The box was closed by a cover allowing limited gaseous exchange with the exterior, and was heated to a temperature of about 550 C. in a furnace for about 2 hours. In this manner, most of the volatile substances were eliminated. After cooling, a pile of very light felt was obtained, about 20 mm. thick. The felt was made electrically conductive by placing it in a furnace in a current of xylene-saturated nitrogen and heating to a temperature of approximately 850 C. for about 1 hour. The direction of flow of the gas was reversed after half an hour, in order to obtain a substantially symmetrical deposit of pyrolytic carbon.

The felt was then treated as in Example 3, except that the nickel was electrolytically deposited without vibration.

EXAMPLE 5 Nickel felts prepared according to any one of the preceding examples were subsequently treated in the vapour phase with chromium, aluminium, titanium or alloys of these metals, and satisfactory products were obtained.

EXAMPLE 6 The method of Examples 1 and 2 was followed except that the carbon skeleton was prepared not from cotton fibres but from viscose (dissolved and spun cellulose) fibres giving additional advantages in that the skeleton fibres were extremely regular (whereas cotton fibres have constrictions where breaks may occur) resulting in greater strength. However, the viscose fibres had a larger diameter (about 30 microns instead of about 5-7). Consequently, viscose may be used to obtain greater mechanical strength, but it is advisable to use cotton if it is desired to have a large expanded surface (for example for accumulator plates and the like).

What we claim is:

1. A method of preparing a fibrous metal material, which comprises, firstly, preparing an electrically conductive carbon skeleton, having a general shape corresponding to that of the finished material which it is desired to obtain, by subjecting carbon elements to pyrolytic heat treatment at a temperature of from 800 to 850 C. in a neutral atmosphere comprising a carrier gas containing a small proportion of a hydrocarbon, said hydrocarbon being liquid at normal conditions of temperature and pressure and said small proportion being obtained automatically by bubbling said carrier gas through said liquid hydrocarbon at normal temperature and pressure, said heat treatment being pursued for a time sufficient to form conductive pyrolytic bridges joining said carbon elements, secondly, depositing on said carbon skeleton a metal or alloy, the electropositivity of which is not greater than 0.7 and, thirdly, eliminating said skeleton by oxidation at a high temperature of at least 500' C. leaving the deposited metal or alloy only, in said fibrous form.

2. A method according to claim 1, wherein said depositing of the metal or alloy on the carbon skeleton is effected by chemical deposition.

3. A method according to claim 1, wherein said depositing of the metal or alloy on the carbon skeleton is effected by electrolytic deposition.

4. A method according to claim 1, wherein said neutral atmosphere is taken from the group consisting of nitrogen and argon.

5. A method according to claim 1, wherein the amount of hydrocarbon in the neutral atmosphere is from 1 to 5 percent by volume.

6. A method according to claim 1, wherein said hydrocarbon is xylene.

7. A method according to claim 1, wherein said elements contain a sufficiently high proportion of carbon to leave a residue of at least 20% by weight of carbon after said pyrolytic heat treatment.

8. A method according to claim 1, wherein said skeleton is of a form selected from the group consisting of a fibre, felt, ribbon and sheet of fabric.

9. A method according to claim 1, including the further step of vapor treatment of the prepared material with at least one metal element which is introduced inside the material so as to increase the corrosion resistance thereof or to improve the mechanical properties thereof.

10. A method according to claim 9, wherein said metal element is taken from the group constituted by chromium, aluminuim, titanium and an alloy of at least two of these metals.

11. A method according to claim 1, wherein the metal or alloy deposited on the carbon skeleton is taken from the group constituted by nickel, cobalt, iron, copper, silver and an alloy of the these metals.

12. A method according to claim 1, wherein the oxidation operation at a high temperature for eliminating the skeleton is performed in a hydrogen atmosphere containing a suitable proportion of water vapor such that the carbon is burnt without the metal being oxidised, the treatment being performed at a temperature of from 800 to 1100 C. for from 20 to hours, so that the treated product, if it has been subjected to mechanical working, is sintered as a result of the treatment.

13. A- method according to claim 1, wherein the oxidation operation at a high temperature for eliminating the skeleton is performed in air at a temperature comprised between 500 and 700 C. for a time comprised between 20 and 5 hours respectively, so that the carbon is elfectively eliminated without excessive oxidation of the metal deposit.

14. A method according to claim 1, including a mechanical working step, wherein, after the skeleton has been removed, the product is sintered and de-oxidised in a hydrogen atmosphere.

15. A method according to claim 1, comprising forming the carbon skeleton of non-interlocking fibres and felting the metal fibres obtained by treatment similar to that used in paper manufacture, before eliminating the carbon skeleton.

References Cited UNITED STATES PATENTS 3,071,637 1/19-63 Horn et a1 DIG. l X 3,087,233 4/ 1963 Turnbull 75DIG. 1 X 3,385,915 5/1968 Hamling 23-354 X CARL D. QUARFORTH, Primary Examiner R. E. SCHAF-ER, Assistant Examiner US. Cl. X.R.

29-482; 75DIG. l, 200, 212, 222 

